This invention relates to estimation of fluid flow, and more particularly to a chronic or acute measurement of blood flow in a blood vessel.
There are many applications in clinical and research medicine in which measurement or estimation of volumetric blood flow within a blood vessel is desirable. One method of making such measurements uses ultrasonic Doppler techniques to measure blood flow velocity and thereby estimate volumetric blood flow. Velocity of an object is often measured using the Doppler effect Single frequency ultrasonic energy is transmitted into an area of tissue containing the blood flow to be measured. This insonification of the area is typically referred to as illumination. Resulting ultrasonic energy is reflected, or backscattered, from the illuminated area Energy reflected from moving targets, such as fluid and blood cells, will be shifted in frequency from the illuminating frequency according to the well-known Doppler effect The Doppler shifted frequency provides a measure of the blood flow velocity.
In clinical and research applications, it is often necessary to study blood flow for an extended period of time. Thus, in ambulatory living organisms, such as animal or human subjects, there is a need in the art to provide a battery-powered ultrasonic Doppler blood flow meter for measuring blood flow velocity for an extended period of time, allowing a human or animal patient freedom of movement during the study and minimizing the need for supervision by the clinician. There is also a need in the art to provide a small, low-power ultrasonic Doppler blood flow meter that is suitable for implantation in a human or animal subject. There is a further need in the art to provide an implantable ultrasonic Doppler blood flow meter that maintains adequate signal-to-noise (SNR) ratio for accurate velocity estimation.
The present invention includes a method and apparatus for estimating blood flow or blood flow velocity in a blood vessel over a period of time. According to the method, at least part of the measurement circuits used to estimate blood flow are automatically activated only during the time an estimate is being obtained. At least part of the measurement circuits are automatically deactivated during the time an estimate is not being obtained. These steps are performed repeatedly to provide a sequence of blood flow estimates forming a blood flow waveform indicative of blood flow. More than one estimate is typically required to obtain a waveform representative of the blood flow.
The steps of activating and deactivating at least part of the measurement circuits is repeatedly performed sufficiently frequently, either periodically or at irregular intervals, such that the blood flow waveform substantially represents the variable blood flow. Power to at least a portion of the measurement circuits is reduced or interrupted while the measurement circuits are deactivated.
Measurement of blood flow can be obtained through various blood flow measurement techniques, including: continuous wave (CW) Doppler flow measurement, pulsed Doppler flow measurement, laser Doppler flow measurement, transit time flow measurement, thermal dilution flow measurement, electromagnetic flow measurement, or other suitable flow measurement technique.
In several embodiments, a basebanded Doppler-shifted signal provides the blood flow estimate. In other embodiments, a blood flow output signal is derived from the basebanded Doppler-shifted signal and provided as the blood flow estimate.
Thus, the present invention provides a strobed blood flow meter, such as an implantable strobed ultrasonic Doppler blood flow meter, having reduced average power consumption, which is advantageous for reducing battery size, extending battery life, and improving signal-to-noise ratio.